The Digital Light Processing (DLP) projector is a projection display having a special light source modulation, which is a new projection system developed by the Texas Instruments (TI). The most important character that the DLP projector has is that it is a full digital reflective projector, which not only projects a finer image but also efficiently reduces the size and the weight of the projector. Since the DLP projector has a simple structure, it is more advantageous in the development of the micro projector.
The miniaturization of the projector should start from the optical system, in which the light source and the optical engine have to be miniaturized, the optical path is reduced thereby, and the panel and the optical element also need to match the reduced size. It is an important issue in the miniaturization of the projector to achieve the miniaturization of the projector and to suppress the noise.
The DLP™ technology is operated based on a micro electrical mechanical element, i.e. the digital micromirror device (DMD) chip. The DMD chip has three layers including an optical micromirror layer at the top, a mechanical supporting layer at the middle and an electronic controlling layer around at the bottom. The optical micromirror layer is a two-dimension array composed of individually controlled micromirrors, each of which micromirror is supported by a yoke joined with a series of torsion hinges and arranged in the mechanical supporting layer, and the micromirror is controlled by a corresponding underlying static random access memory (SRAM) cell in the electronic controlling layer. Each micromirror represents one pixel of an imaged to be projected in the projection screen. The micromirrors on the DMD microchip may receive the data word line represented by the electrical signal and generate the optical word line output.
The structure of the optical system of the DLP projector is shown in FIG. 1, wherein the optical system consists of a light source 10, a color wheel 12, an integrator 14, a condenser 16, a reflective mirror 18, a digital micromirror device (DMD) 20 and a camera lens 22. As shown in FIG. 1, the camera lens 22 is assembled by a plurality of lens. The beams generated from the light source 10 is collected by a concave mask, pass through the color wheel 12 to be the light with different colors, unified by the integrator 14, condensed by the condenser 16 to enter the reflective mirror 18, and then incident to the DMD 20. The DMD 20 digitally process an image to be associated with the incident lights and reflects the lights to the camera lens 22, thereby the lights entering the camera lens 22 are projected on the screen 24.
By deflecting of the micromirrors on the DMD, the incident light may be reflected at different angles and the bright and dark effects of the light spots are achieved. Generally, the rotation angle of the DMD ranges between +10 to −10 degree. When receiving an On state signal, the micromirrors in the DMD rotate about +10 degrees to make the reflected light enter the camera and being projected on the screen so as to form an On state light spot. While the DMD receives an Off state signal, the micromirrors in the DMD rotate about −10 degrees to make the reflected light enter the range outside the camera and do not generate light spot on the screen, i.e. the Off-state. Besides, the reflected light is in a flat state if the DMD does not rotate or rotates at 0 degree. Referring to FIG. 1, when the light 26a becomes the light 26b via the rotation of the DMD 20 and enters into the camera lens 22, the light is projected to the effective area of the screen 24. However, when the DMD 20 rotates with a specific angle and reflects the light 28a to be light 28b, the light 28b does not enter the camera lens 22 but forms an ineffective area outside the screen 24.
The smaller the size of the projector is, the closer the condenser approaches the DMD. At this time, most of the off state light and the flat state light would be reflected by the condenser. In FIG. 1, the light 28b is reflected by the condensing element 16 to become the light 28c, and the light 28c causes a serious stray light outside the screen after it passes through the camera. The stray light is a reflection phenomenon, which forms a bright area at the ineffective area outside the screen and affects the image quality of the projection. For resolving the problem caused by this stray light, one solution is extinction of the surrounding elements of the DMD to decrease the strong reflection by increasing the roughness of the surfaces of the elements. The extinction merely makes the surrounding of the DMD darker but cannot eliminate the reflection completely. Another solution is to enhance the flatness of the mirror surface of the DMD for decreasing the unnecessary stray light greatly, but it results in a higher cost. A further solution is to compose a baffle around the DMD to block the light, such a baffle still cannot eliminate the reflection completely because the stray light is still projected to the surrounding of the DMD. Moreover, the baffle is required to be assembled additionally and thus increases the complexity of the system assembly.
In view of the drawbacks of the prior art, it is a need to provide a projection device which has an easy assembly design and a low cost for decreasing the stray light phenomenon under the existing DLP structure. The summary of the present invention is described below.